The first blazar observed at z > 6

Abstract

We present the discovery of PSO J030947.49+271757.31, the radio brightest (23.7 mJy at 1.4 GHz) active galactic nucleus (AGN) at z>6.0. It was selected by cross-matching the NRAO VLA Sky Survey and the Panoramic Survey Telescope and Rapid Response System PS1 databases and its high-z nature was confirmed by a dedicated spectroscopic observation at the Large Binocular Telescope. A pointed Neil Gehrels S wi f t Observatory XRT observation allowed us to measure a flux of ∼3.4×10 −14 erg s −1 cm −2 in the [0.5-10] keV energy band, which also makes this object the X-ray brightest AGN ever observed at z>6.0. Its flat radio spectrum (α νr <0.5), very high radio loudness (R>10 3 ), and strong X-ray emission, compared to the optical, support the hypothesis of the blazar nature of this source. Assuming that this is the only blazar at this redshift in the surveyed area of sky, we derive a space density of blazars at z∼6 and with M 1450Å < -25.1 of 5.5 +11.2 −4.6 ×10 −3 Gpc −3 . From this number, and assuming a reasonable value of the bulk velocity of the jet (Γ=10), we can also infer a space density of the entire radio-loud AGN population at z∼6 with the same optical/UV absolute magnitude of 1.10 +2.53 −0.91 Gpc −3 . Larger samples of blazars will be necessary to better constrain these estimates.

Parsec-scale properties of the radio brightest jetted AGN atz> 6

“…In the VLBI images we do not find any evidence for a counterjet, which indicates that this AGN is seen under a relatively small viewing angle, as expected if PSO J0309+27 is a blazar (Belladitta et al 2020). To estimate the possible ranges of viewing angles θ and of the bulk velocity in terms of speed of light (β bulk ), it is possible to use the ratio of jet to counter-jet brightness J (Giovannini et al 1994, see Eq.…”

“…In order to clearly assess the origin of this disagreement, detection and confirmation of blazars at large redshifts is urgently required. A&A 643, L12 (2020) By combining the NRAO VLA Sky Survey catalogue (NVSS, Condon et al 1998, in the radio), the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS PS1, Chambers et al 2016, in the optical), and the AllWISE Source Catalogue (WISE, Wright et al 2010;NEOWISE, Mainzer et al 2011, in the mid-infrared) and using the dropout technique, we discovered PSO J0309+27, which we spectroscopically confirmed to be at a redshift z = 6.10 ± 0.03 (Belladitta et al 2020). It is the brightest AGN in the radio band discovered to date above redshift 6 (23 mJy as measured with the NVSS).…”

“…Lobanov 2010;Lister et al 2013). Before Belladitta et al (2020), only seven blazars were known at z > 5, the most distant being at z = 5.47 (Romani et al 2004). This is essentially due to the rarity of this type of radioloud AGN: we estimated ∼2 × 10 −4 blazars every square degree at z > 5.5 and with flux densities higher than a few mJy, corresponding to about two to three blazars between z = 5.5 and z = 6.5 that should be detectable in currently available widefield surveys (Caccianiga et al 2019, but see also Padovani et al 2017).…”

“…In addition to the NVSS observation, PSO J0309+27 is also detected in the TIFR GMRT Sky Survey (TGSS, 64.2 ± 6.4 mJy, Intema et al 2017) at 147 MHz and at 3 GHz by the VLA Sky Survey (VLASS, 12.0 ± 1.2 mJy, Lacy et al 2019). On the basis of the multi-wavelength observed properties, in particular those at X-ray energies, Belladitta et al (2020) classified PSO J0309+27 as a blazar.…”

Observations by GMRT at 323 MHz of radio-loud quasars atz > 5

“…A significant number of quasars have been discovered at z > 5 (e.g., Fan et al 2006;Jiang et al 2015;Mazzucchelli et al 2017). However, only a few of these are classified as radio-loud quasars (e.g., Anderson et al 2001;Fan et al 2001;Sharp et al 2001;Romani et al 2004;McGreer et al 2006McGreer et al , 2009Willott et al 2010;Zeimann et al 2011;Yi et al 2014;Bañados et al 2018;Belladitta et al 2020). High-redshift radio-loud quasars are interesting because their prominent radio jets should directly relate to the activity of their central SMBHs.…”